Optimal design of the transmission-way of mechanical stress throughthe interface between a joint prosthesis and bone tissue together with the criterion formulation for the assessment of the fixation method

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Failure criteria of ultra-high molecular weight polyethylene (UHMWPE) should be clarified for the strength design of joint prostheses. In the present study, we performed compression tests of UHMWPE plates together with finite element (FE) analysis under conditions similar to those within a total knee prosthesis and discuss the failure criteria.Flat plates made of conventional or cross linked UHMWPE were molded as test pieces. Those thicknesses were ranging from 2 mm to 8 mm. The compression test using a loading machine was performed in a bath filled with saline solution at 37 degrees centigrade. A metallic sphere 50 mm in diameter was used for the indenter. The load was applied to 6 kN under a crosshead speed of 2 mm per minute. Flow pressure was calculated after the test as a ratio of the impressio area and the maximum load. Deformation of the plate was estimated by displacement at the edge. Additionally, the FE analysis was performed on a virtual model using the same conditions as in … More the experiments.The values of the flow pressure were measured as a mean of 32.8 MPa, SD 2.3 MPa for conventional UHMWPE, and as a mean of 37.3 MPa, SD 0.9 MPa for cross-linked UHMWPE. The displacement of the UHMWPE plate increased with decreasing thickness of the plate. The displacement estimated by FE analysis was similar to that of theexperimental findings. Furthermore, shear strain in the UHMWPE plate of FE model increased with the displacement. It was concluded that the failure criteria of joint prostheses must be presented by not only the stress but also the strain in a UHMWPE insert.Stress in bone tissue must be remained after the press-fit-fixation of a implant. The residual stress must bring on the biological response around the implant. The effect has not been sufficiently discussed, yet, because the residual stress is stationary and unlike the physiological mechanical conditions.In the present study, we developed a method to configure stationary stress field in a rat femur using a pin with the hyper elasticity. As the results, it was observed that bone hypertrophy emerged at the fringe of the femur under six weeks stationary load from the pin inside bone marrow. Less